1. Field of the Invention
The present invention concerns a method and a device to regulate a multi-axis manipulator, in particular a robot.
2. Description of the Prior Art
Modern industrial robots are predominantly position-regulated. For this purpose, desired joint positions are generated in an interpolator and supplied to individual joint regulators in the individual actuators of the robot. However, if a desired position is not reached—for example due to an obstacle or an inaccurate environment modeling in the path plan—a pure position regulation increases the drive forces until either manipulated variable limits are exceeded—which leads to the deactivation of the controller—or the robot forcibly approaches the desired position, and thereby possibly damages an obstacle, a component, a tool or itself.
An illustrative example is spot welding with an electrode holder. The electrode holder should be pressed with a predetermined contact pressure force at predetermined points of a component. However, if the component is not in the position on which the welding pose is based in the planning, but rather is shifted in the closing direction of the electrode holder, a merely position-regulated robot would forcibly push the welding electrodes into the predetermined positions and thereby damage the module and one of the electrodes, while the other electrode possibly has no contact with the component. Even if no damage occurs, impermissible forces that could reduce the quality of the welding spot to the point of failure would occur.
In order to solve such problems, it is known to provide targeted passive flexibilities in the robot structure, for example with a feature known as a “Remote Center of Compliance”. Due to the task-specific rigidity and flexibility tendencies, however, this is not usable for different application cases. An actively controllable tool connection—for instance an electrode holder with floating bearing that can be adjusted in different compensation positions by means of compressed air or servomotors—must be matched to the respective application case and, like the passive flexibility, requires additional device expense.
Therefore, for a long time force regulations have been tested in research, wherein antiparallel force pairs (i.e. torques) are generally also designated as forces; by force regulation of the type known as force moment regulation (“FMR”). One concept used in industry as “FTCtrl” (“force torque control”) is to divide the space in which movement occurs into position-regulated and force-regulated sub-spaces using a selection matrix. Another known technique is parallel position and force regulation with superimposition of the respective manipulated variables. A further known technique is impedance regulation in which positions and forces are linked via force rules, in particular spring-damper (shock absorber) mass models. For example, there is an overview by H.-B. Kuntze in “Regelungsalgorithmen für rechnergesteuerte Industrieroboter” [“Regulation algorithms for computer-controlled industrial robots”], Regelungstechnik, 1984, S. 215-226 or by A. Winkler in “Ein Beitrag zur kraftbasierten Mensch-Roboter-Interaktion” [“A contribution to force-based human-robot interaction”], Dissertation, TU Chemnitz, 2006.
However, the realization of these approaches regularly runs into difficulties in practice. For example, a force regulation in Cartesian space in order to show a flexibility along one Cartesian direction—for instance along the closing direction of the electrode holder in the above application case—requires a high computing effort with correspondingly slow regulation response. For force regulation of all axes it is problematic to determine the respective proportion of the forces acting on the robot from the motor currents, since the forces can only be imprecisely reconstructed due to gearing conversions, friction and noise.